Kotlin Ktor Cheat Sheet 🚀

A quick reference guide for setting up and working with Ktor in Kotlin.

1. Setting Up Ktor

Install the required dependencies in build.gradle.kts:

dependencies {
    implementation("io.ktor:ktor-server-core:2.x.x")
    implementation("io.ktor:ktor-server-netty:2.x.x")
    implementation("io.ktor:ktor-server-content-negotiation:2.x.x")
    implementation("io.ktor:ktor-serialization-kotlinx-json:2.x.x")
}

2. Defining Routes

fun Application.routes() {
    routing {
        get("/health/liveness") {
            call.respond(HttpStatusCode.OK, HealthResponse(2.0))
        }

        post("/authentication/login") {
            call.respond(HttpStatusCode.OK)
        }
    }
}

3. Handling Request Parameters

get("/api/scores/riskGroup") {
    val person = with(call.request.queryParameters) {
        Person(
            email = get("email") ?: throw MissingRequestParameterException("email"),
            firstName = get("firstName") ?: throw MissingRequestParameterException("firstName"),
            lastName = get("lastName") ?: throw MissingRequestParameterException("lastName"),
            postcode = get("postcode") ?: throw MissingRequestParameterException("postcode")
        )
    }
    call.respond(HttpStatusCode.OK, person)
}

4. Middleware (Plugins)

fun Application.installPlugins() {
    install(ContentNegotiation) {
        json()
    }
    
    install(StatusPages) {
        exception<MissingRequestParameterException> { call, cause ->
            call.respond(HttpStatusCode.BadRequest, "400: ${cause.message}")
        }
    }
}

5. Singleton Mock Database

object MockDatabase {
    private val data = mutableMapOf<String, String>()

    fun insert(key: String, value: String) { data[key] = value }
    fun get(key: String): String? = data[key]
    fun delete(key: String) { data.remove(key) }
    fun clear() { data.clear() }
}

Usage:

MockDatabase.insert("user1", "Alice")
println(MockDatabase.get("user1")) // Alice
MockDatabase.delete("user1")

6. JWT Authentication

Generating a JWT

fun generateJWT(userId: String): String {
    val secretKey = "my-secret-key"
    return Jwts.builder()
        .setSubject(userId)
        .setIssuedAt(Date())
        .setExpiration(Date(System.currentTimeMillis() + 3600000)) // 1 hour
        .signWith(Keys.hmacShaKeyFor(secretKey.toByteArray()), SignatureAlgorithm.HS256)
        .compact()
}

Validating a JWT

fun validateJWT(jwt: String): String {
    val secretKey = "my-secret-key"
    return Jwts.parserBuilder()
        .setSigningKey(secretKey.toByteArray())
        .build()
        .parseClaimsJws(jwt)
        .body
        .subject
}

7. Hashing with SHA-256

fun hashSHA256(input: String): String {
    val bytes = MessageDigest.getInstance("SHA-256").digest(input.toByteArray())
    return bytes.joinToString("") { "%02x".format(it) }
}

Usage:

val hashed = hashSHA256("password123")
println(hashed) // Prints hashed value

8. Mocking with coEvery for Unit Tests

Using MockK to mock service calls:

@Test
fun `given fraud score found when GET scores then respond OK`() = runTestApplication {
    coEvery { fraudScoreService.getFraudScore(any()) } returns FraudScore(100)

    val response = client.get("/api/scores") {
        url { parameters.append("email", "test@example.com") }
    }

    response.status shouldBe HttpStatusCode.OK
}

9. Controller Handler

val healthController: suspend PipelineContext<Unit, ApplicationCall>.(Unit) -> Unit = {
    delay(200)
    call.respond(HttpStatusCode.OK, HealthResponse(number = Math.PI))
}

Usage in Routes:

get("/health/liveness", healthController)

10. Handling JSON Responses

data class ApiResponse(val message: String)

get("/json") {
    call.respond(HttpStatusCode.OK, ApiResponse("Hello, JSON!"))
}

11. Handling Enums (RiskGroup)

enum class RiskGroup { A, B, C, D, E, F }

val riskGroup = RiskGroup.values()[fraudScore.value] // Convert ordinal to Enum

Conclusion

This Kotlin Ktor Cheat Sheet provides a quick reference for setting up Ktor, defining routes, handling authentication, hashing data, testing, and working with singleton databases. 🚀